Coaxial cavity resonator with separate controls for frequency tuning and for temperature coefficient of resonant frequency adjustment

ABSTRACT

A temperature compensated tuning apparatus for a co-axial resonant cavity comprises a telescopic tuning member having a first section and a second section, the sections having different co-efficients of thermal expansion and being connected by a coefficient of expansion adjusting screw which has an internally threaded bore engaged with the first section and an externally threaded part engaged with the second section; a capacitive disc mounted co-axially on the end of the first section within the cavity is maintained in electrical contact with the second section by means of resilient contact members while permitting a telescopic relative motion of the first section with respect to the second section. The first member is made of an alloy of 36 percent nickel and 64 percent iron while the second member and the co-efficient expansion screw are made of brass or copper. The arrangement provides a cavity having an adjustable positive thermal co-efficient of resonant frequency which is useful for compensating for the negative co-efficient of frequency of a Gunn diode oscillator.

TJite States atent Johnson 45] May 15, 1973 RESONANT FREQUENCYADJUSTMENT [75] Inventor: Albert Henry Johnson,

Christchurch, England [73] Assignee: Minister of Aviation Supply in HerBrittannic Majestys Government of the United Kingdom of Great Britainand Northern Ireland, London, England [22] Filed: Apr. 13, 1971 [21]Appl. No.: 133,691

[52] US. Cl. ..333/82 B, 333/82 ET [5]] Int. Cl. ..I-l01p 1/30, H01p7/04 [58] Field of Search ..333/82 BT, 83 T, 333/82 B, 83 R [56]References Cited UNITED STATES PATENTS 2,109,880 3/1938 Dow ..333/82 BT2,124,029 7/1938 Conklin et a1. ..333/82 BT 2,173,908 9/1939 Kolster..333/82 BT 2,205,851 6/1940 Hansel] ..333/82 BT 2,533,912 12/1950 Bels..333/82 BT 2,637,782 5/1953 Magnuski.. .....333/82 B 2,716,222 8/1955Smullin ..333/83 T 3,160,825 12/1964 Derr 333/83 T FOREIGN PATENTS ORAPPLICATIONS 333,746 12/1958 Switzerland ..333/82 BT OTHER PUBLICATIONSGoud, P.A. Cavity Frequency Stabilization with Compound TuningMechanisms, Microwave Jr. 3-1971, pp. 55-56, 58.

Primary ExaminerRudolph V. Rolinec Assistant Examiner-wm H. PunterAttorney-Hall, Pollock & Vandesande [57] ABSTRACT A temperaturecompensated tuning apparatus for a co-axial resonant cavity comprises atelescopic tuning member having a first section and a second section,the sections having different co-efficients of thermal expansion andbeing connected by a co-efficient of expansion adjusting screw which hasan internally threaded bore engaged with the first section and anexternally threaded part engaged with the second section; a capacitivedisc mounted co-axially on the end of the first section within thecavity is maintained in electrical contact with the second section bymeans of resilient contact members while permitting a telescopicrelative motion of the first section with respect to the second section.The first member is made of an alloy of 36 percent nickel and 64 percentiron while' the second member and the co-efficient expansion screw aremade of brass or copper. The arrangement provides a cavity having anadjustable positive thermal co-efficient of resonant frequency which isuseful for compensating for the negative co-efficient of frequency of aGunn diode oscillator.

4 Claims, 1 Drawing Figure v 115i; V %4 7 $17 18 i l u 7 2s 1 l5 i 14COAXIAL CAVITY RESONATOR WITH SEPARATE CONTROLS FOR FREQUENCY TUNING ANDFOR TEMPERATURE COEFFICIENT OF RESONANT FREQUENCY ADJUSTNENT BACKGROUNDOF THE INVENTION The present invention relates to tuning mechanisms forwaveguide structures, particularly applicable to resonant cavities ofthe coaxial type, which may be used in microwave engineering apparatusfor generating or for operating on high frequency electromagnetic waves.Particularly, but not exclusively, the present invention relates tocoaxial resonant cavities in which a Gunn diode is used to provide amicrowave generator.

In known microwave generators using a Gunn diode mounted in a coaxialresonant cavity, it is difficult to maintain the frequency of thegenerator constant with varying temperature, because Gunn diodes exhibitlarge negative frequency/temperature coefficients and there is widerandom variation between diodes when mounted in typical resonantcavities. Gunn diodes, as presently produced, exhibitfrequency/temperature coefficients in the range 0.5 to 2.5 MHz/C whenoperated in typical resonant cavities as microwave generators at X-bandfrequencies (8 to 12 GHz). Because of the large value and range ofnegative frequency/temperature coefficients, compensating circuitrywhich may be quite complicated, often has to be incorporated in theoscillator circuit.

An object of the present invention is to provide a tuning mechanism fora coaxial resonant including a mechanism for producing an adjustablepositive frequency/- temperature coefficient, to compensate for thetemperature dependence of typical Gunn diodes.

SUMMARY OF THE INVENTION According to the present invention there isprovided a tuning mechanism for a waveguide structure comprising atelescopic tuning member having a first section and a second section andeach made of a material having a different coefficient of thermalexpansion to that of the other, connected by a temperature coefficientadjustment screw having a first threaded part engaged with screw threadon the first section and a second threaded part engaged with a screwthread on the second section; and contact means disposed to make anelectrical contact, or an effective short-circuit at the operatingfrequency of the waveguide structure, between the surfaces of the firstsection and the second section which will be required to act ascurrentcarrying surfaces within the waveguide structure, whilepermitting a telescopic relative motion of the first section withrespect to the second section.

The waveguide structure may be an electromagnetically resonant cavity.

The contact means may comprise a contact member attached to one of thesections of the tuning member and having a set of resilient contactfingers making contacts to the other section. Alternatively, it may be aseparate part having two sets of resilient contact fingers makingcontact to the first section and the second section respectively. It maybe possible, in some applications of the invention, to use some form ofchoke coupled non-contacting arrangement in place of a set of contactfingers, but at the operating frequencies of most Gunn diode oscillatorcircuits this alternative will probably be impractical.

BRIEF DESCRIPTION OF THE DRAWING An embodiment of the present inventionwill now be described, by way of example only, with reference to theaccompanying drawing which is a sectional view of I a coaxial resonantcavity utilizing the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A coaxial resonant cavity 1comprises a metal block 2 and a metal post 3 one end of which iselectrically connected to the block 2. The block 2 has a threaded hole18 through a face opposite to and in axial alignment with the post 3. Anexternally threaded tuning screw 4 having an external thread 5, aninternal bore 6 and an internal screw thread 7 is screwed into the hole18. An adjusting screw 8, for adjusting the frequency/temperaturecoefficient, has an external thread 9 which matches and engages thethread 7 on the tuning screw 4, and an internal bore 10 which has a step19 at its lower end. The step 19 has, at its center, a threaded hole 11whose thread has the same pitch as the external thread 9 of theadjusting screw 8. All the aforementioned parts are made of brass or asimilar material. A plunger comprises a stem 12 and a disc 14 both ofwhich are made of the 36 percent nickel 64 percent iron alloy known asINVAR (Registered Trade Mark), the stem 12 having a screw thread 13along its length. The screw thread 13 matches and engages the screwthread of the hole 11 in the adjusting screw 8. The tuning screw 4, theadjusting screw 8 and the plunger stem 12 and its disc 14 are allcoaxial. A hole 16 in the base of the tuning screw 4 allows clearancefor the stem 12. The diameter of the internal bore 10 of the adjustingscrew 8 similarly provides clearance around the stem 12 except at thestep 19. A locknut 17 is engaged with the thread 9 and may be tightenedagainst the tuning screw 4 to prevent unintentional adjustment of theadjusting screw 8. A contact spring 15 is provided between the base ofthe tuning screw 4 and the back of the disc 14 to take up any backlashin the engagement of the threads 11 and 13 and to provide alow-impedance path for radio frequency currents between the disc 14 andthe tuning screw 4. A key 25 protrudes from the end of the tuning screw4 into a keyway cut longitudinally in the stem 12 to prevent the plungerl2, 14 from rotating when the adjustment screw 8 is rotated, whileallowing movement of the plunger 12, 14 with respect to the tuning screw4. A Gunn diode 24 attached in a suitable manner to a metal mountingmember 20 contacts the post 3 at a predetermined distance from the topface of the post 3. The mounting member is surrounded by an electricallyinsulating sheath 23 which is externally threaded. The whole assembly isscrewed into a hole 21 in the block 2. A locating hole I 22 in the post3 is used to locate one end of the Gunn diode 24. Electrical connections(not shown) to the Gunn diode 24 are conveniently made through the metalmounting member 20 and the metal block 2 of the cavity 1. A coaxialoutput connection (not shown) is provided to draw electromagnetic energyfrom the cavity 1.

In operation, the component parts are assembled as shown in the drawingand the tuning screw 4 is adjusted to produce the required resonantfrequency of the cavity. This frequency is determined in part by thegeometry of the cavity 1 and the post 3, by the capacitance between thelower face of the disc 14 and the top face of the post 3 and by theefiective capacitance of the Gunn diode 24. Electrically, a Gunn diodemay be regarded as a capacitance in parallel with an oscillator and theelectrical equivalent circuit of the resonant cavity and Gunn diode maybe regarded as a parallel tuned circuit comprising an inductance, due tothe post 3, a capacitance, due to the gap t between the disc 14 and thepost 3, and another capacitance due to the Gunn diode 24, all of whichare in parallel with an oscillator. The value of the capacitance betweenthe disc 14 and the post 3 is inversely proportional to the distance t.

It can be shown that if the disc 14 were rigidly attached to the base ofthe tuning screw 4, the variation of the capacitance with temperaturewould be a function of the initial value of the distance 1 and of thecoefficient of linear expansion of the cavity material.

That is to say:

where C, is the capacitance at some temperature T at which temperaturethe distance 2 is taken as t AC is the change in capacitance due to achange in temperature AT.

a1 is the coefficient of linear expansion of the cavity material.

It can also be shown that the change Af of the resonant frequency fo duesolely to the change in capacitance is given by Assuming that (Af/f isnegligible, which in many typical applications will be a reasonableapproximation, an approximate expression for the variation in frequencydue to the change in capacitance is Af/AT= m, j,

Hence for a cavity resonator made entirely of brass with an initialresonant frequency f of 8GHZ, the value of Af/AT will be approximately80 Kl-lz/C. (Since 01 for brass is 2 X 10*). The value of Af/ATisdependent on the geometry of the cavity and the operating frequency.

In embodiments of the invention as herein describedand illustrated thetemperature coefficient adjusting screw 8 forms an attachment betweenthe screw 4 and the stem 12, by the engagement of screw threads 11 and113 on members 8 and 12 and of screw threads 7 and 9 on members 4 and 8,holding the screw 4 and the stem 12 in a fixed relationship to oneanother at a position Lo above the post 3 and furthermore, the distanceL, is adjustable while the gap t at temperature T between the disc 14and the post 3 may be set at a predetermined size.

For convenience it is assumed that the tuning screw 4, the adjustingscrew 8 and the cavity block 2 and post 3 are all made of brass. Thestem 12 and the disc 14 are made of the 36 percent nickel, 64 percentiron alloy known as INVAR (Registered Trade Mark). The coefficient oflinear expansion (12 of INVAR is taken to be 1 X 10*.

The top of the post 3 may be taken as a reference point becauseeverything below it expands at the same rate. The distance L, betweenthe top of the post 3 and the base of the adjustment screw expands withtemperature at a rate given by AL/AT 111 L,

where AL is the change in L due to a change AT in temperature.

Similarly, the rate at which the part of the plunger 12, 14 below theadjustment screw 8 expands is given by AX/AT= a2 X,

where X, is the length of the plunger below the adjustment screw at atemperature T and AX is the change in X due to a change in temperatureAT. Clearly L X, t

The resulting change At in the distance t between the lower face of thedisc 14 and the top face of the post 3 is given by:

At (L al X,,a2)AT At/t (L al X a2)/t AT,

so that the term (L al X 012), can be considered to be the effectivecoefficient of expansion of the telescopic arrangement, and it willhereinafter be written ae. Corresponding to equation (3), for thecomposite telescopic arrangement, the variation of frequency due to thecapacitance variation caused by the temperature change AT will now begiven by lffi, SGHZ, L, 0.5 and t =0.02" l.8MHz/C.

Furthermore, the value of the frequency/temperature coefficient Af/ATcan be adjusted by varying the length X by turning the screw 8. Forinstance it may be adjusted to compensate for the temperature dependenceof a chosen Gunn oscillator diode.

It will be apparent to those skilled in the art that the invention maybe used in connection with waveguide filters and the like.

What I claim is:

1. A coaxial cavity resonator incorporating a telescopic inner conductorwhich comprises,

a tubular tuning screw having an external thread engaged with thestructure of one end of the resonator, and having an internally threadedbore,

a temperature-coefficient adjusting screw having external and internalthreads of substantially the same pitch, the external thread beingengaged and then Af cooperating with the internal thread of the bore ofthe said tubular tuning screw,

a stem having an external thread engaged and cooperating with theinternal thread on the said temperature coefficient adjusting screw,said stem being formed of a material having a coefficient of thermalexpansion different from the coefficient of thermal expansion of thematerial of the said tuning screw,

a plunger mounted on said stem so as to form a tuning capacitance withthe structure of the resonator which capacitance can be varied byadjusting the said tuning screw,

and means for preventing relative rotation of the said stem with respectto the said tuning screw.

2. A coaxial cavity resonator incorporating a telescopic inner conductoras claimed in claim 1, and further including a cylindrical post axiallyaligned with the said inner conductor and having one end thereofattached to the structure of the said resonator, the other end of saidcylindrical post forming the main capacitative component of the saidresonator in conjunction with said plunger of the said inner conductor.

3. A coaxial cavity resonator as claimed in claim 2 wherein the saidtelescopic inner conductor also comprises a contacting means located andmaking electrical contact between the said plunger and the said tuningscrew.

4. A coaxial cavity resonator as claimed in claim 3 wherein thecontacting means comprises two sets of resilient fingers formed on asingle metallic member and mounted so that one of the sets makeselectrical contacts with the plunger while the other set makeselectrical contacts with the tuning screw.

1. A coaxial cavity resonator incorporating a telescopic inner conductorwhich comprises, a tubular tuning screw having an external threadengaged with the structure of one end of the resonator, and having aninternally threaded bore, a temperature-coefficient adjusting screwhaving external and internal threads of substantially the same pitch,the external thread being engaged and cooperating with the internalthread of the bore of the said tubular tuning screw, a stem having anexternal thread engaged and cooperating with the internal thread on thesaid temperature coefficient adjusting screw, said stem being formed ofa material having a coefficient of thermal expansion different from thecoefficient of thermal expansion of the material of the said tuningscrew, a plunger mounted on said stem so as to form a tuning capacitancewith the structure of the resonator which capacitance can be varied byadjusting the said tuning screw, and means for preventing relativerotation of the sAid stem with respect to the said tuning screw.
 2. Acoaxial cavity resonator incorporating a telescopic inner conductor asclaimed in claim 1, and further including a cylindrical post axiallyaligned with the said inner conductor and having one end thereofattached to the structure of the said resonator, the other end of saidcylindrical post forming the main capacitative component of the saidresonator in conjunction with said plunger of the said inner conductor.3. A coaxial cavity resonator as claimed in claim 2 wherein the saidtelescopic inner conductor also comprises a contacting means located andmaking electrical contact between the said plunger and the said tuningscrew.
 4. A coaxial cavity resonator as claimed in claim 3 wherein thecontacting means comprises two sets of resilient fingers formed on asingle metallic member and mounted so that one of the sets makeselectrical contacts with the plunger while the other set makeselectrical contacts with the tuning screw.